1. Field of the Invention
The present invention generally relates to the field of electro-mechanical sensors for measuring an applied force, and more specifically to a tunneling-tip sensor and a photolithography method for fabricating the sensor.
2. Description of the Related Art
One method for sensing physical quantities such as linear or rotational acceleration, or acoustic or hydrophonic pressure is to provide a flexible member that flexes in response to an applied force and measures the amount of flex electrically. Conventional micro-mechanical techniques for achieving the transduction include capacitive coupling, piezoresistive sensing and piezoelectric sensing. However, none of these techniques are inherently as sensitive as tunneling tip transduction.
In tunneling tip sensors, a bias voltage is applied across a flexible counter electrode and a tunneling tip with a sufficiently small gap between the two components to induce a tunneling current to flow. The tunneling current I.sub.T is given by: I.sub.T .varies.V.sub.B exp (-.alpha.h.sqroot..phi.), where V.sub.B is the bias voltage, .alpha. is a constant, h is the electrode-to-tip separation and .phi. is the work function. As the applied force changes, the separation between the electrode and the tip changes and modulates the tunneling current, which varies by approximately a factor of three for each angstrom (.ANG.) of electrode deflection. Thus, tunneling tip detectors can provide a much greater sensitivity and a larger bandwidth than previous method of detections and still provide easily measurable signals.
For the specific application of the sensor as an accelerometer, the deflection distance x=ma/k, where m is the electrode's mass, k is the electrode's spring constant and a is the acceleration. The effective bandwidth of the accelerometer is determined by its resonant frequency ##EQU1## Since tunneling tip techniques are more sensitive to deflection, the accelerometer's mass can be relatively small, and thus its bandwidth can be larger than the capacitive coupling and piezoresistive devices.
A tunnel tip sensor and its fabrication method are disclosed in Kenney et al., "Micromachined silicon tunnel sensor for motion detection," Applied Physics Letters Vol. 58, No. 1, Jan. 7, 1991, pages 100-102. A flexible folded cantilever spring and a tunneling tip are formed on a first silicon wafer by etching completely through the wafer to form a proof mask pattern. The pattern defines an inner rectangular area that is suspended by first and second folded flexible members that extend from the outer portion of the wafer to the inner rectangle. The cantilever spring and tunneling tip are formed by thermally evaporating gold through respective shadow masks onto the patterned wafer to define respective contacts on the wafer's outer portion that extend therefrom along the respective folded members to a rectangular mass and a tip on the inner rectangular portion of the wafer. The cantilever spring and tunneling tip are physically connected by the proof mask which allows them to deflect in unison in response to an applied force but are electrically isolated from each other. A second wafer is etched to define a hole approximately the size of the cantilever spring's rectangular mass and a tunneling counter electrode. A third wafer is etched to define a deflection counter electrode approximately the size of the cantilever spring's rectangular mass. The 200 .mu.m thick wafers are then pinned or bonded together by placing the first wafer with the cantilever spring and tip face up on the bottom, and placing the second wafer with the tunneling counter electrode suspended above the tip at a separation of approximately 50 .mu.m and the hole above said cantilever spring's mass. The third wafer is placed on top of the second with the deflection counter electrode disposed above the hole such that it is suspended above the cantilever spring. The three wafers are mechanically attached with alignment pins or epoxy and electrically connected to a separate analog feedback circuit.
A control voltage is applied between the deflection counter electrode and the cantilever spring to provide an attractive force that brings the tip close enough to the tunneling counter electrode for a bias voltage applied between the tunneling counter electrode and the tip to induce a tunneling current of approximately 1.3 nA. The cantilever spring and tip deflect in response to an applied force to modulate the tunneling current. The cantilever spring provides the mass required to produce a measurable deflection and the desired sensitivity for the accelerometer. The analog feedback circuit compares the measured tunneling current to a setpoint, and modulates the control voltage to adjust the separation between the tunneling counter electrode and the tip to maintain a constant current. The modulated control voltage provides an output proportional to the applied force.
Although this tunneling tip sensor provides a more sensitive and compact sensor than the other conventional sensors, its fabrication method and structure have several deficiencies. Fabricating three separate 200 .mu.m wafers and bonding them together produces sensors that are approximately 4 cm.sup.2 in area, with manufacturing yields of approximately 5%. These relatively large size and low yield sensors are very expensive to manufacture. The tip-to-tunneling electrode separation is nominally 50 .mu.m and requires a control voltage of approximately 200 volts to bring the tip close enough to the tunneling electrode to induce the tunneling current. The high voltage levels are not compatible with other TTL or CMOS circuitry and variations in the separation cause large variations in the required control voltage. The cantilever spring has a mass of 30 mg, which restricts the resonant frequency to approximately 200 Hz and a bandwidth that is comparable to those of other conventional techniques. The accelerometer fails to achieve a larger bandwidth because of its relatively large mass. The design of the cantilever spring makes the sensor sensitive to off-axis (x or y-axis) forces and large temperature coefficients and drift. Furthermore, when the feedback circuit is turned off of a large shock can deflect the spring, causing the tip to impact the tunneling counter electrode and be damaged due to the relatively large spring mass.